The present invention relates to an output compensating device of an image sensor and, more specifically, to an output compensating device for compensating for variations in pixel outputs of a MOS type image sensor.
Japanese Laid-Open Patent Publication No. 2000-329616 discloses a conventional MOS transistor type image sensor using a number of light sensor circuits each representing a unit pixel, which circuit comprises, as shown in FIG. 1, a photodiode PD operating as a photoelectric converting element for producing a sensor current proportional to the quantity of incident light Ls falling thereon, a transistor Q1 having a logarithmic output characteristic in a weak inverse state for converting the sensor current produced in the photodiode into a voltage signal Vpd by using its sub-threshold region characteristic, a transistor Q2 for amplifying the voltage signal Vpd and a transistor Q3 for outputting a sensor signal in accordance with a timing pulse of a readout signal Vs and which circuit is characterized by its wide dynamic range obtained by giving the output a logarithmic characteristic, thereby achieving the high sensitivity of detecting a light signal. In addition, the light sensor circuit is provided with a means for changing a drain voltage VD of the transistor Q1 to a value lower than a normal value for a specified period to remove a charge accumulated in a parasitic capacitor C of the photodiode PD to initialize the circuit. The light sensor circuit can thus obtain a voltage signal Vpd corresponding to the quantity of incident light Ls even if the sensor current absurdly changed, thereby eliminating the possibility of occurrence of an afterglow of the pixel even at a small quantity of incident light.
As shown in FIG. 3, the above-described light sensor circuit may output a signal with a logarithmic characteristic at a normal quantity of sensor current corresponding to a normal quantity of incident light to a photodiode but it may not maintain the logarithmic output characteristic and have a substantially linear output characteristic at a decreased sensor current due to a delay of charging the parasite capacitor of the photodiode. In FIG. 3, WA represents a region of responding with a non-logarithmic characteristic output and WB represents a region of responding with a logarithmic characteristic output.
However, the conventional image sensor using the above-described light sensor circuits forming a matrix of pixels cannot be free from structure-derived variations in output characteristics of pixel signals as shown in FIG. 4, which variations shall be compensated respectively. In FIG. 4, lo denotes a dark sensor current corresponding to a dark current flowing in each photodiode PD with no light falling thereon. Therefore, the conventional image sensor compensates for variations of the output characteristics of the pixel circuits in the following ways. First, the image sensor shall conduct offset compensation of all pixel circuits so that outputs from all pixel circuits at the dark current Io (with no incident light to the photodiodes) are equal to each other. Next, it shall conduct gain compensation for variations of the output characteristics of the pixel circuits so that output characteristics of all pixel circuits in a bright state with incident light to the photodiodes have the same inclination. Alternatively, the variations of output characteristics of all pixel circuits may be cancelled by reversing the order of conducting the above two kinds of compensating operations.
However, it is difficult to perform in practice the accurate offset and gain compensation for variations in output characteristics of respective light sensor circuits by switching off and on illumination of the image sensor to bring the sensor circuits to have the same output level at the same dark current and the same bright current because a light source itself may have variations in luminance and cannot evenly illuminate each of the pixels, frequently being turned ON and OFF.
Furthermore, conducting the output compensation of a number of image sensors at the same time requires the use of a number of light sources. This means the need for increasing the size of the production installation.
In addition, the image sensor cannot be free from having variations in output characteristics of pixel circuits, which may be caused from uneven aging of the light sensor circuits composing the image sensor. In other words, the image sensor must be subject to periodical offset and gain compensation for possible variations in the output characteristics of its light sensor circuits to maintain the same necessary levels of output characteristics of respective pixels at dark and bright currents by switching on and off the illumination.
Furthermore, the image sensor using a matrix of the light sensor circuits each representing a unit pixel suffers variations in output characteristics of light sensor signals Vo, which may result from structure-derived variations and temperature characteristic variations of respective light sensor circuits. This means that the image sensor cannot produce accurate outputs in a dark state and a bright state when taking video without compensation for the above-described variations in the output characteristics of the image sensor.
In consideration of the above, Japanese Patent Applications No. 2000-404931, 2000-404933, 2000-75035 by the assignee of this application disclose an image sensor which has been compensated for output characteristics of its light sensor circuits in the final inspection process before shipment in such a way that actual output variation values of each light sensor circuit are measured, an offset compensation value and a gain compensation multiplier corresponding to the measured values are read out from a compensation table stored in a memory and then the sensor circuit is offset- and gain-compensated for values determined by calculation, thus achieving the accurate compensation for variations in output characteristics of respective light sensor (pixel) circuits of the image sensor.
However, the image sensor comprising a number of light sensor circuits each representing a unit pixel, in which a sensor current proportional to incident light is produced in a photoelectric converting element and converted into a voltage signal by using a MOS type transistor having logarithmic output characteristic in a weak inverse state, still involves such a problem that the image sensor cannot be free from the occurrence of variations of output characteristics of respective sensor (pixel) circuits during the use of the image sensor after initial compensation. This causes degradation of the image taken by the image sensor.